new StressTransform class to convert various stress tensors
new StressTransform class to convert various stress tensors

- .get_cauchy_from_2pk() so far

vlukes/sfepy,rc/sfepy,rc/sfepy,sfepy/sfepy,vlukes/sfepy,lokik/sfepy,RexFuzzle/sfepy,sfepy/sfepy,BubuLK/sfepy,rc/sfepy,RexFuzzle/sfepy,BubuLK/sfepy,lokik/sfepy,olivierverdier/sfepy,BubuLK/sfepy,lokik/sfepy,olivierverdier/sfepy,RexFuzzle/sfepy,sfepy/sfepy,RexFuzzle/sfepy,olivierverdier/sfepy,lokik/sfepy,vlukes/sfepy